α, β, γ, n Sources and Detection

What it shows:

Radiations originating from atomic and nuclear processes are classified into four types:

charged particulate radiation consisting of
1. heavy charged particles (α)
2. fast electrons (β)
uncharged radiation consisting of
3. electromagnetic radiation (γ, x-ray)
4. neutrons (n)

The interaction processes of each type of radiation explain their penetrability through matter, their difficulty or ease of detection, and their danger to biological organisms. The interactions of these radiations with matter are unique and the purpose of the demonstration is to show how the physics determines the detection techniques in each of the four types.

How it works:

We first describe the sources presently available and follow with how their radiations are detected.

Radioactive Sources:
1. Alpha particles are 4He nuclei and are generally emitted by very heavy nuclei containing too many nucleons to remain stable. The alpha particle source is 241Am (5.49 MeV (85%) and 5.44 MeV (13%) α's) which also gives off γ and x-rays. 1 It has a half-life of 433 yr.

2. Beta particles are fast electrons (or positrons) emitted as a result of the decay of a neutron (or proton) in nuclei which contain an excess of the respective nucleon. Nuclides that decay directly to the ground state are "pure beta emitters." 290Sr/90Y is such a source, emitting a continuous spectrum of fast electrons up to a maximum energy of 0.546/2.27 MeV. 3 The half life is 27.7 yr/64 hr.

3. Gamma rays are electromagnetic radiation emitted by excited nuclei in their transition to lower nuclear energy levels (following beta decay). Like the discreet atomic energy levels which give rise to specific photon energies, the well defined nuclear states are the source of specific gamma ray energies. We use the two most common of such sources: 137Cs, which gives off a 662 keV gamma, and 60Co (1.173 and 1.332 MeV gammas). Half lives are 30.2 and 5.26 yr, respectively. 4

4. There are no natural isotope sources of neutrons, but by mixing an alpha-emitting isotope with a suitable target material, neutrons can be produced through a nuclear reaction. 226Ra/Be is such a combination. Under bombardment of alphas, beryllium undergoes a number of reactions which leads to the production of free neutrons (see Neutron Activation of Silver for details). The neutron energy spectra of all alpha/Be sources are pretty much similar: 1-13 MeV max, with about 5 MeV average. Two neutron sources are available with neutron flux of 3.2×105 n/sec and 6.6×104 n/sec. 5 The sources contain 22.5 mg and 10 mg of radium respectively, and give off an appreciable gamma ray background that must be contended with. The half life of Ra-226 is 1602 yr.

Detectors:
The net result of the radiation interaction in a wide variety of detectors is the production of electric charge (either directly or indirectly) within the detector. The detectors we generally use are the following.

1. To detect the alpha particles only, a spark detector 6 is used because it is insensitive to the γ and x-rays also given off by the 241Am source. The detector consists of a course grating of fine wires in close proximity (≈ 1 mm) to a ground plane. The principle of operation is identical to a Geiger-Müller counter (ionization detector) except that the gas (air) is at atmospheric pressure. Several thousand volts applied to the wires results in a strong electric field. The alphas ionize the air and these ion/electron pairs are accelerated by the strong field. Secondary ion/electron pairs are produced by collisions and further interactions quickly produce an avalanche, which manifests itself as a spark. (The detector does not respond to betas; energy being equal, they are not as heavily ionizing a particle as alphas.) A microphone placed near the detector allows the entire audience to hear the spark as a "click"; the actual spark may be shown with a close-up video camera.

2 & 3. For the detection of betas and gammas, a Geiger-Müller counter 7 (ionization detector) is used. It consists of a cylindrical container with conducting walls and a thin end window. The cylinder is filled with a noble gas (usually argon) at less than atmospheric pressure. Along its axis is suspended a conducting wire to which a positive high voltage (usually about 900 volts) is applied through a resistor. When radiation enters the cylinder, electron-ion pairs are created (either directly or indirectly, depending on whether the radiation is a charged particle or neutral). The electrons are accelerated towards the anode (wire) and the ions toward the cathode (cylinder wall). This small current produces a voltage drop across the anode resistor, which in turn is capacitively coupled to a suitable amplifier. The output of the amplifier is registered on an analog meter as well as audible clicks from a loudspeaker.

4. Because neutrons produce no direct ionization events, neutrons must be detected through nuclear reactions which result in heavy charged particles. First the neutrons are slowed down (thermalized) by elastic collisions with light nuclei - typically this is some hydrogenous material such as water, paraffin (CH2) or, as in this case, polyethylene. One of the more popular reactions for the conversion of slow neutrons into directly detectable particles is the 10B(n,α) reaction which goes as follows:

Our BF3 proportional detector 8 is based on this reaction; boron trifluoride serves as both the target for slow neutrons as well as the proportional gas used to detect the alphas (natural boron consists of about 20% of the isotope 10B). The proportional detector is surrounded by a cylinder (8 cm dia × 17 cm long) of polyethylene to thermalize the fast neutrons.

Setting it up:

The detectors can all be set up on the lecture bench -- video projection is recommended as the readouts of the instruments are small.

1. The alpha spark detector needs a separate high voltage power supply. The 10 kV Beckman Instr. model 6010-M is useful for this purpose. Raise the voltage until spontaneous discharges begin (about +4.8 kV) and then reduce the voltage slightly. The alpha emission rate is only 2.19×105 α/min 9 and therefore the Am-241 source must be held quite close (≤ 1 cm) to the detector to take advantage of the 1/r2 intensity increase.

2 & 3. No voltage adjustments are necessary on the G-M detector. The thin end-window must be used for the detection of betas. High energy gammas happily make it through the thicker side walls of the G-M tube.

4. The bias voltage on the Ludlum 2200 should be adjusted to 1750. For purposes of neutron detection only, we recommend using the weaker of the two neutron sources. The source should be positioned well away from the front row of the audience and heavily shielded with lead bricks. Remove the source from its lead pig only long enough to perform the measurement. A word of caution concerning radiation safety: observe the three rules. (1) Shielding -- use the lead bricks for protection. (2) Time -- keep exposure time to a minimum. (3) Distance -- use the 1/r2 law to your advantage.

The Sr-90, Co-60, and Cs-137 sources are relatively weak (µCi or fractions thereof) and pose no safety concern if handled properly with "minimal" safety procedures. Millicurie sources, on the other hand, are 1000 times stronger and must be handled in a safe manner.

Comments:

Everyone should be introduced to radioactive isotopes, radiations, and detection methods. Rating ***

References:

R.D. Evans, The Atomic Nucleus, (Krieger, NY, 1982)
G.F. Knoll, Radiation Detection and Measurement, 2nd ed, (Wiley, NY, 1989)

1 The gamma ray energies are 59.5 keV (85%) and 26.4 keV (2.4%); The L x-rays are from Np decay and have energies of 20.8 (4.9%), 17.8 (19.4%) and 13.9 keV (13.3%).
2 Decays to an excited state of the nucleus additionally produce gamma rays in the de excitation process.
3 The energy spectrum is a continuum because the available energy for the decay is shared between the beta (or positron) and the antineutrino (neutrino).
4 Note that Cs-137 is also a source of internal conversion electrons with energies of 656 and 624 keV.
5 Calibrated by the NRC of Canada (3/13/56) and Eldorado Mining of Canada.(3/30/51), respectively.
6 Cenco cat. no. 71248 spark counter
7 Ludlum Measurements model 177 ratemeter with model 44-7 G-M tube.
8 Ludlum model 42-9 operated at 1750 volts. Power supply and readout are provided by Ludlum model 2200 Scaler/Ratemeter.
9 Calibrated 1/14/80 by New England Nuclear